1. Field of the Invention
The present invention relates to a method and apparatus for a permeable membrane gas separation system. More particularly, it relates to the prevention of condensation in such systems and method of protecting the membranes for longevity and control of gas purity. Longevity depends on the cleanliness of the feed air. The typical filtration system found in competitive systems often only meet the minimum requirements from the membrane manufacturers. This leads to premature aging of the membranes and a shortening of their useful life. The feed air purification system in the present invention is designed to provide air quality better than ordinary ambient air and better than systems designed for breathing air. The result is an extended useful life of the membrane separator(s).
2. Description of the Prior Art
The use of polymeric fiber membranes have been used for many years for the production of nitrogen gas from ordinary compressed air. In addition, whereas the addition of heat is not a requirement for the membrane to operate, there has always been the risk of airborne contaminants reaching the membrane fibers causing either a temporary or permanent reduction of capacity or total failure of the membrane. One of the causes widely known in the nitrogen generation industry is that of water vapor condensing in the air purification train, especially in the carbon tower and in the membrane separator due to ambient conditions which allow the moisture in compressed air to condense. Various methods are and have been in use to eliminate this problem. One such method is to use a separate compressed air dryer, either refrigerant type or regenerative adsorbent type to remove a significant amount of water prior to entry into the feed air purification train, or pre-filtration for a better description. The use of air heaters to superheat the air and lowering relative humidity is not new to the nitrogen generation industry. The air heater, in addition to lowering relative humidity and carrying any remaining moisture in the vapor state to the membrane, is a common practice in the industry. The heated air also causes the membrane to perform in a predictable manner and flow of nitrogen at a given oxygen content increases directly with an increase in temperature or decrease in temperature. So pre-heating the feed air to the membrane serves two basic purposes. First is to keep moisture from condensing in the air purification train or process stream, especially in the carbon bed and membrane and secondly, to provide for a stable production of nitrogen under various ambient temperature conditions. The source of heat for this purpose traditionally has been from electric heaters, steam heat exchangers, air-to-air and air-to-oil heat exchangers using heat of compression as the heat source to provide a cost effective means to provide such heat to the membrane. Some prior methods require insulation of membranes and/or housing the membranes in a separated heated enclosure, whereas some other methods require insulation of the piping and components of the air purification system and most require insulating the membranes, which is expensive and makes servicing the system's components difficult. Although these prior methods have their merits, in practice they can be quite expensive and intrusive and in some cases detrimental to the operation and reliability of the air compressor. In prior methods, a single heater or heat exchanger has been utilized for the entire BTU required.
The practice of using moisture separators, coalescing filters, carbon towers, and particle filters to protect the membranes from solid particles, condensed water and oil, oil aerosol, oil vapor and other hydrocarbons present in the feed air stream from oil flooded air compressors is commonly used in the industry. Some systems use multi-staging of filters in their design, some use single coalescing filters and most systems size the filtration system to where the operating pressure loss across the pre-filtration system is quite significant. In the science of membrane separation it is common to all manufacturers of membranes that production of gas is directly related to pressure at the inlet to the membrane separators; The higher the air pressure, the more production (flow) at a given purity (percent of oxygen in product). Increasing the number of stages of filtration and lowering the operating pressure loss is desirable from a membrane protection standpoint, but size constraints, economics and other factors can be prohibitive. Multi-staging of filtration, by definition, means an increasing number of stages, whereby each stage is highly efficient at its design grade, but with rougher grades followed by finer grades where the resulting efficiency of the combination is greatly improved. While these existing pre-filtration systems are adequate, there is a definite need for a pre-filtration system that meets high performance of multi-staging AND does it with a minimum of filter housings and pressure loss, thereby allowing a higher inlet pressure to the membrane(s) which will allow for a higher flow rate of nitrogen at a given nitrogen purity.
Gas purity controls have ranged in designs to a simple manual valve, valves with back pressure regulator, differential pressure regulator measuring the differential across an orifice that operate a control valve, automated controls that use the output of oxygen from an oxygen analyzer to control a valve to match the oxygen set point. There are also valves that are self contained that will control the flow and purity, but have a high built in pressure drop across the valve. The problem with a manual valve is that under varying downstream pressure, the flow and resulting purity of the product cannot be maintained. A manual valve with a back pressure regulator has some degree of control of flow under fluctuating downstream pressure, but adjustment of the two valves can be complicated and the flow/purity accuracy is not very desirable. The differential pressure regulator, orifice and control valve combination works well in controlling the purity, but there are so many components, the system is both confusing and expensive. The automated controls using PID (proportional-integral-derivative) controls, oxygen analyzer analog output and control valve is a very expensive system with many components and takes a relatively long time to tune initially and on starting the machine, takes too long to reach the controlled purity and in some cases will oscillate due to upsets in the system. While these systems have proven adequate to one degree or another, a simpler solution to purity control is needed.